This invention relates to an improved paint powder supply device for use in electrostatic powder painting apparatus as extending from a paint powder tank to a painting gun and adapted to effect a uniform discharge of paint powder through the painting gun and thus to create a painted surface having a uniform paint film thickness.
A conventional paint powder supply device has various problems. First, the paint powder tank has the following problems. Heretofore, a fluidized type paint powder tank has been used as the tank of the electrostatic paint powder apparatus.
In such a paint powder tank, a porous resin plate is provided at the bottom of the tank. Compressed air is supplied into the tank through the porous resin plate to stir and dehumidify the paint powder in the tank. The density of the paint powder in the tank changes with its amount in the tank.
For example, as the amount of paint powder in the tank decreases, i.e. as the level of the paint powder in the tank becomes lower, the density of the paint powder decreases correspondingly.
Thus, if the paint powder in the fluidized type powder tank is drawn out by a discharge device such as an injector, the discharge rate changes according to the paint powder level in the tank as shown by line a in FIG. 1.
Thus, it was impossible when using such a conventional fluid type paint powder tank to discharge the paint powder at a uniform rate.
Secondly, there have been the following problems in drawing out the paint powder from the paint powder tank. A conventional device for drawing the paint powder out of the paint powder tank comprises a discharge pipe provided at the lower part of the paint powder tank and a pinch valve attached to the discharge pipe and adapted to close the flow line when air is supplied thereto. The paint powder in the paint powder tank can be discharged by opening and closing the pinch valve.
In this type of device, if the discharge of paint powder is stopped for a long time by the pinch valve, the paint powder left in the part of the discharge pipe upstream of the pinch valve may become moist so that the discharge pipe can become clogged with the paint powder at that part. This makes it difficult for the paint powder to drop smoothly when the pinch valve is opened and thus to supply the paint powder smoothly.
Thirdly, a paint supply device for supplying the paint powder in the paint powder tank to the painting gun had the following problems.
In a conventional paint supply device, an injector is directly attached to the side or top of the paint tank containing the paint powder. The injector is coupled to the painting gun through a hose. The paint powder in the paint tank is drawn out by the injector together with air. The air is used to send the paint powder to the painting gun.
The discharge rate through the painting gun is determined by the air pressure and amount of air in the injector. In order to create a painted surface having a uniform film thickness, the discharge rate through the painting gun has to be uniform.
But even if the air pressure and amount of air in the injector are unchanged, when the amount of paint powder in the paint tank decreases, the air content in the paint powder increases and the pressure changes according to the thickness of the paint powder, so that the amount of paint powder sucked by the injector decreases. This in turn causes a decrease in the rate at which the paint powder is discharged through the painting gun. Thus, it was difficult to form a paint film having a uniform thickness. This may result in transparent paint films or defective painted goods.
If paint powder sticks to the inner surface of the hose extending from the injector to the painting gun or if the hose is closed with foreign matter, a pressure loss will occur in the hose. This reduces the amount of paint sucked into the injector, thus changing the discharge rate of the paint powder.
Thus, in a conventional paint supply device having an injector attached to the paint tank so as to directly aspirate the paint powder from the paint tank, it was difficult to discharge the paint powder through the painting gun at a uniform rate.